In recent years, semiconductor devices having thin film transistors (TFTs) and thin film diodes (TFDs) on the same substrate and electronic devices including such semiconductor devices have been developed. As a method of manufacturing such semiconductor devices, forming semiconductor layers for the TFTs and the TFDs of the same crystalline semiconductor film formed on a substrate has been proposed.
Patent Document 1 discloses an image sensor in which an optical sensor unit utilizing a TFD and a driver circuit utilizing a TFT are formed on the same substrate. In Patent Document 1, semiconductor layers of the TFT and the TFD are formed by crystallizing an amorphous semiconductor film formed on the substrate.
When the TFT and the TFD are formed in a unified manner on the same substrate as described above, the size of the semiconductor device can be reduced. Additionally, it leads to a significant cost advantage such as the reduction in the number of parts required. Further, it also makes it possible to provide a product with new additional features, which would not be available by conventional part combinations.
On the other hand, Patent Document 2 discloses a technology of forming a TFT of crystalline silicon (crystalline silicon TFT) and a TFD of amorphous silicon (amorphous silicon TFD) on the same substrate using the same semiconductor film (amorphous silicon film). Specifically, a catalyst element that facilitates the crystallization of amorphous silicon is added only to the region that is to be an active region of the TFT in the amorphous silicon film formed on a substrate. Then, a heat treatment is performed so as to form a silicon film in which the crystallization has occurred only in the region that is to be an active region of the TFT, and the region that is to be a TFD remains amorphous. By using such a silicon film, a crystalline silicon TFT and an amorphous silicon TFD can be formed on the same substrate with ease.
Further, Patent Document 3 discloses forming an optical sensor TFT that functions as an optical sensor and a switching TFT that functions as a switching element using the same semiconductor film (amorphous silicon film). More specifically, the channel region of the optical sensor TFT is formed of an amorphous silicon film, and the source and drain regions of the optical sensor TFT and the active region of the switching TFT are formed of a crystalline silicon film. Also, by making the amorphous silicon film for the channel region of the optical sensor TFT thicker than the crystalline silicon film for the active region of the switching TFT, the sensitivity of the optical sensor is improved.
Patent Document 3 discloses the following method to create the above-mentioned TFTs so that they become different from each other. First, an amorphous silicon film is formed on a substrate. Next, in the photolithography to form island-shape films of the amorphous silicon film, the amorphous silicon film is partially thinned with the half exposure technology using a gray tone mask. In this manner, a plurality of amorphous silicon layers having different thicknesses are provided. After that, these amorphous silicon layers are irradiated with laser light such that the thinned regions of the amorphous silicon layers (the regions that become the source and drain regions of the optical sensor TFT and the active region of the switching TFT) are crystallized, and the region that has not been thinned (the region that becomes the channel region of the optical sensor TFT) remains amorphous.